Spemann

1
Spemanns Organizer Primary Embryonic Induction
2
Primary Induction must be understood in terms
of movement of
cells

• Mesodermal cells migrate up wall of blastocoel
• Become arranged as loose sheet immediately under
  region of
• ectoderm that later becomes neural plate
• Dorsal mesoderm induces local ectoderm to form
  neural plate

3
Additional anterior axis
Formation of Tail Structures
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Nervous system is induced during gastrulation
Ventral tissue not yet determined at time of
transplantation!
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Induction is region specific
What is the molecular nature of the inducer?
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Neuralization may depend on inhibition of the BMP
pathway!
Factors secreted by Spemanns organizer
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Wnt and FGF neuralize ectoderm but precede
Organizer signalling (not localized to organizer
mesoderm)!
Wnt expression downregulates BMP mRNA expression
during gastrulation TCF (a transcriptional corep
ressor is essential!
8
Additive pathways may regulate BMP expression to
neuralize ectoderm

• FGF signalling (blue) is
• ubiquitous higher levels in
• animal (A) marginal (M) zones?
• Low levels of BMP mRNA (yellow)
• found in A and M zones.
• increase in early gastrula
• Active Wnt (orange)- on dorsal side
• BMP protein antagonists (white)
• expressed in organizer
• Neural effector genes (pink) begin
• expression BMP mRNA is cleared

Either bind to BMP4 or its receptor
9
Midterm Exam Questions Friday March 1, 2002
One of these questions will be chosen! All
answers must be supported with examples from the
literature.

• Why is embryonic induction a key principle in
  understanding
• development?
• How do receptor and signal transduction
  mechanisms contribute
• to our understanding of the establishment
  of spatial information
• in the embryo pre- and post-fertilization.
• Gametogenesis is regulated at so many levels its
  amazing that
• anyone has children! Support or refute the
  statement!